1. Field of the Invention
The invention relates to the production of hydrogen and oxygen by the electrolysis of water and, more particularly, to the production of hydrogen by the photoelectrolysis of water using solar radiation.
2. Description of the Prior Art
Two areas that have recently received considerable attention are solar energy conversion and the use of hydrogen as the ultimate fuel for the distribution and interconversion of energy (the hydrogen economy). A coupling of solar energy conversion with the hydrogen economy may produce a solution to both energy resource depletion and environmental pollution problems. Such a coupling may be effected by photolysis, in which sunlight is used to directly decompose water into hydrogen and oxygen. Preferably, semiconductor electrodes may be employed to facilitate photolytic decomposition by a process known as photoelectrolysis. A simple model for such a process can be described in terms of conventional photovoltaic devices which comprise an n-p junction. In photoelectrolytic devices, the n-p junction is replaced by an n-electrolyte-p (or metal) junction. Electron-hole pairs are generated by the absorption of light in both semiconductor electrodes. The electron-hole pairs are separated by the semiconductor-electrolyte junction barrier and are injected at the respective electrodes to produce electrochemical oxidation and reduction reactions.
For an n-type electrode, holes combine with hydroxyl ions (OH.sup.-) to produce an anodic oxidation reaction; the reverse process occurs at a p-type or metal electrode where electrons combine with protons (H.sup.+) to produce a cathodic reduction reaction. The net effect is a flow of electrons from the n-electrode to the p-electrode resulting in reduction at the latter (H.sub.2 formation) and oxidation at the former (O.sub.2 formation).
The attractiveness of photoelectrolysis is that it solves the energy storage problem which plagues the practical implementation of solar energy conversion, and at the same time it has the potential for providing an inexpensive source of hydrogen. However, the current use of single crystal electrodes, which are inherently expensive, and economic considerations relating to other aspects of the cell design, limit its current use. New and more efficient cell designs and materials are required in order to realize the potential of photoelectrolysis.
A photoelectrolysis cell consisting of thin film n- and p-type semiconducting electrodes of different compositions has been disclosed in U.S. Pat. No. 3,925,212, issued Dec. 9, 1975 to D. Tchernev. In this cell, however, the n- and p-type electrodes are arranged in a "spaced" configuration such that sunlight must sequentially pass first through a large bandgap electrode before reaching a small bandgap electrode. The light that is absorbed in the smaller bandgap electrode consists only of light not absorbed in the large bandgap electrode. This cell configuration suffers from the disadvantage that the large bandgap electrode must be supported on a highly transparent and highly conducting substrate. Such substrates are rare, expensive and difficult to manufacture with both sufficiently high transparency to sunlight and sufficiently high conductivity to produce photoelectrolysis cells of high efficiency.
Further, in the cell configuration in U.S. Pat. No. 3,925,212 the areas of the n- and p-type electrodes are equal. However, for optimum efficiency, the relative areas of the n- and p-type electrodes must be adjusted such that equal rates of formation of electron-hole pairs are produced when the cell is illuminated with sunlight.